A polymer electrolyte fuel cell is a power-generating apparatus by causing an electrochemical reaction while supplying hydrogen to a fuel electrode and oxygen to an oxidant electrode. The fuel electrode and the oxidant electrode are bonded to the respective surfaces of a solid polymer electrolyte membrane containing perfluorosulfonic acid acting as an electrolyte
The following reactions take place in the respective electrodes.Fuel Electrode: H2→2H++2e−Oxidant Electrode: ½O2+2H++2e−→H2O
A higher output power of 1 A/cm2 or more can be obtained at ambient temperatures and atmospheric pressure in accordance with these reactions in the polymer electrolyte fuel cell.
In the fuel electrode and the oxidant electrode, there are provide mixtures consisting of carbon particles supporting a catalyst substance and a solid polymer electrolyte. Generally, these mixtures are applied on backing layers such as carbon paper acting as layers for diffusing fuel gas. These two electrodes sandwiching the polymer electrolyte membrane are thermally bonded under pressure to configure the fuel cell.
In the fuel cell thus configured, hydrogen gas supplied to the fuel electrode reaches to the catalyst after passing through fine pores in the electrode to be converted into hydrogen ions by releasing the electrons. The released electrons are guided to an external circuit after passing through the carbon particles and the solid polymer electrolyte in the fuel electrode and flow into the oxidant electrode through the external circuit.
On the other hand, the hydrogen ions generated on the fuel electrode reach to the oxidant electrode through the polymer electrolyte in the fuel electrode and the solid polymer electrolyte membrane sandwiched between both the electrodes, and then form water by means of the reaction with oxygen supplied to the oxidant electrode and the electrons flowing from the external circuit in accordance with the above reaction formula. As a result, the electrons flow in the external circuit from the fuel electrode toward the oxidant electrode to generate electric power.
In order to improve the characteristics of the fuel cell having the above configuration, a better adhesion is important at the interfaces between the electrodes and the polymer electrolyte membrane. That is, the higher electro-conductivity of the hydrogen ions generated through the electrode reaction is required at the interfaces between them. An inferior adhesion at the interface increases the electric resistance due to the decrease of the conductivity of the hydrogen ions to cause a reduction of the cell efficiency.
While the fuel cell using hydrogen as the fuel has been described heretofore, the research and the development regarding the fuel cell using an organic liquid fuel such as methanol have been extensively conducted in recent years.
In the meantime, the electrical losses generated in the cell are required to be low as much as possible for improving the output power characteristics of the fuel cell. The main electric losses include those due to resistance over voltage, activation over voltage and concentration over voltage, and the loss of fuel not reacted in the fuel electrode.
In order to decrease the activation over voltage among these, the use of a catalyst substance having a higher catalyst activity is important in the fuel electrode and the oxidant electrode.
In order to decrease the resistance over voltage, it is important to reduce the respective resistance losses as much as possible including the resistance of a polymer electrolyte membrane, the resistance of a catalyst electrode and a contact resistance.
The resistance over voltage due to the contact resistance includes the contact resistance between a catalyst and a carbon particle supporting the catalyst. Platinum or platinum-based alloy is used as the catalyst for the fuel cell. The platinum-based catalyst having a particle diameter about 2 to 10 nm is supported on the surfaces of the carbon particle by using a wet process such as impregnation. The platinum-based catalyst having a relatively lower bonding force with carbon is a substance which hardly forms a carbide. This is because the aggregation among the catalyst atoms on the carbon particle surface is energetically more stable than the bonding of the platinum-based catalyst with the carbon. Accordingly, when the platinum-based catalyst is supported on the carbon particle surface, the catalyst is generally adsorbed as spherical particles as shown in FIG. 1. The adsorption of the catalyst in the form of the spherical particles reduces the contact area between the catalyst particles and the carbon particles, thereby increasing the contact resistance between the catalyst particles and the carbon particles. That is, the diffusion resistance of electrons generated on the fuel electrode from the catalyst substance to the carbon particle is increased to limit the output power of the fuel cell.
When, as shown in FIG. 1, the catalyst substance is adsorbed in the form of the particles on the carbon particle surface due to the relatively weaker bonding between the catalyst and the carbon particle, this adsorption is responsible for the aggregation of the catalyst particles in addition to the increase of the contact resistance. That is, the catalyst aggregates on the carbon particle surface with the use thereof to increase the particle size when fuel cell is used in the state of power generation because the bonding energy between the platinum and the carbon is lower. Since the redox reaction of the fuel cell takes place on the catalyst surface, the larger catalyst particle reduces the specific surface area of the catalyst to increase the current density. Accordingly, the higher overvoltage due to the aggregation of the catalyst particles generates the output power reduction of the fuel cell.
As described above, it is required in the fuel cell that the contact resistance between the catalyst substance and the carbon particle is reduced, and the aggregation of the catalyst substance after a long period of continuous operation and the output power reduction accompanied thereby are suppressed.
In view of the foregoing circumstance, a technical problem of the present invention is to provide a catalyst-supporting carbon particle, a composite electrolyte and a catalyst electrode for a fuel cell in which the contact resistance between the catalyst substance and the carbon particle supporting the same is lower and the aggregation of the catalyst substance is suppressed, and is to provide a fuel cell having the higher output power and the excellent durability and a method for fabricating the same.